1. Field of the Invention
The present invention relates to an apparatus for precise distance measurement, and specifically to an apparatus that precisely measures a distance between the apparatus and a target.
2. Description of Related Art
U.S. Pat. No. 5,075,878 discloses a laser measurement device for measuring the distance between the device and a target. The laser measurement device uses a zero crossing detection method to calculate the distance. With reference to FIGS. 5A and 5B, the device processes a reflected analog input signal to calculate the distance by the zero crossing detection method, and then outputs a square wave signal. With further reference to FIG. 5B, the duration of the square wave signal is measured by a high frequency signal and then the device will obtain the phase φ.
But the laser measurement device disclosed in U.S. Pat. No. 5,075,878 had some shortcomings. When the reflected input signal includes DC bias signal, a zero point offset is generated in the reflected input signal. The zero point offset injects an error into the zero crossing detection method. Furthermore, a reference signal has noise signals so the device executes the square wave signal with multiple small pulses corresponding to the noises. The small pulses affect the phase accuracy.
The laser measurement device in U.S. Pat. No. 5,075,878 uses many elements and complex circuits and does not have good performance when measuring distance.
The present invention provides an apparatus for precise distance measurement to mitigate or obviate the aforementioned problems.